This invention relates to pumping systems.
A conventional fuel control system for a gas turbine engine comprises an engine driven main fuel pump and a metering control adapted to sense various engine parameters for controlling the rate of fuel flow to the engine's combustion chambers in accordance therewith. Main fuel pumps and existing fuel control systems are generally fixed displacement gear pumps or vane pumps which provide output flows which exceed engine fuel requirements under certain conditions (e.g., high altitude operations). Such main fuel pumps, therefore, necessitate the bypassing of fuel which engenders fuel heating, impairs pumping efficiency and creates other problems such as detracting from the ability of the fuel to cool engine accessories and oil and hydraulic systems without exceeding a maximum safe engine temperature at the burner nozzles.
In order to prevent excessive fuel heating in a bypass loop, various alternative schemes have been proposed or utilized to solve the fuel temperature increase problem. One such problem is the utilization of a variable delivery fuel pump which may be constituted by a positive displacement pump or an impeller pump. If a centrifugal pump alone is employed as the main pumping element in a fuel control system, starting flow requirements mandate that it be of a size sufficient to generate the necessary pressure at low engine speeds and thus at high engine speeds, the generated pressure may be excessive. Also, a centrifugal pump alone is not capable of furnishing the dry lift essential to the proper starting operation of most fuel control systems. The latter considerations therefore normally render a positive displacement pump an indispensable component of the fuel control system.
U.S. Pat. No. 3,851,998 is directed toward a fuel pump assembly for a gas turbine engine incorporating a positive displacement vane pump and a centrifugal pump. In the pump assembly of the patent, the vane pump supplies fuel from start to acceleration to idle speed and the centrifugal pump supplies fuel to the gas turbine engine thereafter. The vane pump disclosed in the patent is of the type having a stationary centrally disposed cam and an annular rotor with radially inwardly directed vanes. High pressure is maintained behind the vanes to prevent the centrifugal forces acting on each of the vanes from moving them radially outwardly and thereby unloading the vane pump. As idle engine speed is approached, internal valving relieves the high pressure behind the vanes, thereby quickly permitting the centrifugal force to overcome the spring load on the vanes and cause them to be retracted to an inoperative position where they do not engage the cam surface. A difficulty with a pump assembly, as illustrated in the aforementioned patent, is that the vanes are subjected to large bearing loads due to the high pressure urging them into engagement with the cam surface. Another drawback of the pump assembly shown in the patent is that valving must be provided, together with a suitable engine speed sensor, to vent the high pressure behind the vanes at the predetermined engine speed.